Vitamin D is one of the most important nutrients for our body, yet many people do not pay enough attention to adequate intake—especially during the winter months. Did you know that 80–85% of the Hungarian population is vitamin D deficient by the end of winter? In this article, we explain why this vitamin is vital, its physiological effects, how to prevent deficiency, and which natural sources can help replenish it. Read on to learn how to keep your body and mind healthy even during the cold months.

Vitamin D, also known as calciferol, is a fat-soluble vitamin that plays a crucial role in numerous physiological processes in the human body. It is primarily involved in bone formation, calcium (Ca) and phosphorus (P) metabolism, and the absorption of these minerals as well as vitamin A. There are two main forms of vitamin D: D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D2 comes from plant sources and fortified foods, while vitamin D3 is produced in the skin under the influence of sunlight and can also be obtained through diet.

Recommended daily intake and units

The recommended daily intake of vitamin D varies by age and individual needs. Generally, the recommended amount is 5–10 micrograms (μg) per day, equivalent to 200–400 international units (IU), since 1 IU of vitamin D equals 0.025 micrograms. It is important to note that vitamin D requirements may differ individually, and in certain cases higher intake may be necessary.

Physiological roles

Vitamin D plays a key role in several vital processes:

• Bone formation: Supports the absorption of calcium and phosphorus in the intestines, contributing to strong bones and teeth.
• Calcium and phosphorus metabolism: Regulates blood levels of these minerals, ensuring proper balance for bone mineralization and muscle function.
• Nutrient absorption: Enhances the absorption not only of calcium and phosphorus but also of vitamin A, which is essential for vision, immune function, and cellular processes.
• Immune support: Helps maintain immune defense and supports the body in fighting infections.
• Anti-inflammatory effect: Contributes to the regulation of inflammatory processes, potentially reducing the risk of chronic diseases.

Deficiency symptoms

AVitamin D deficiency can lead to several health problems, most notably bone-related disorders:

• Rickets: Primarily affects children when bones fail to mineralize properly due to vitamin D and calcium deficiency. This results in soft, weak bones and deformities such as bowed legs. Although rare, rickets may also occur in adults in the form of osteomalacia, characterized by bone weakness, pain, and muscle weakness.
• Osteoporosis: In adults and the elderly, vitamin D deficiency can reduce bone mineral density, increasing fragility and fracture risk. Since vitamin D is essential for calcium absorption and utilization, long-term deficiency may lead to bone loss, spinal deformities, bone pain, and even spontaneous fractures. Osteoporosis particularly affects postmenopausal women but can also occur in men and older adults.

Maintaining adequate vitamin D levels is therefore essential for bone health and long-term physical stability.

Overdose and side effects

Although vitamin D is essential, excessive intake can cause serious health issues. As a fat-soluble vitamin, it is not easily excreted like water-soluble vitamins (such as vitamin C). Instead, excess amounts accumulate in the liver and fatty tissues, potentially leading to toxicity over time.

• Hypercalcemia (elevated blood calcium levels): Excessive vitamin D increases calcium absorption beyond the body’s needs, raising blood calcium levels. Symptoms may include nausea, vomiting, loss of appetite, constipation, muscle weakness, and confusion. Severe cases can result in kidney damage, kidney stones, and heart rhythm disturbances.
• Calcium deposition: Excess calcium may accumulate in blood vessels, the liver, lungs, kidneys, and stomach, potentially causing vascular problems, kidney failure, and organ damage.
• Eye inflammation: Overdose may also cause eye inflammation, redness, and light sensitivity due to calcium-related inflammatory reactions.

Because vitamin D is not easily eliminated, excessive intake over time can be dangerous. It is therefore important to follow recommended daily amounts and consult a physician when adjusting dosage.

Stability and sensitivity

Vitamin D is resistant to heat and air exposure, meaning it does not easily degrade during cooking or storage. However, it is highly sensitive to light, especially ultraviolet (UV) radiation, which rapidly breaks it down. Therefore, vitamin D-rich foods and supplements should be stored in dark, light-protected conditions.

Vitamin D status in Hungary

In Hungary, 80–85% of the population is vitamin D deficient by the end of winter. This is largely because vitamin D synthesis occurs in the skin through sunlight exposure. During winter, sunlight hours decrease significantly, and even when the sun shines, exposure of only the face and hands is insufficient for adequate production. Effective synthesis requires at least one-eighth of the body surface—such as arms and legs—to be exposed to direct sunlight for about 20 minutes daily. Since this is often impractical in winter, supplementation becomes especially important.

The importance of vitamin D supplementation in winter

Reduced sunlight exposure during winter can lower vitamin D levels, leading to various health issues. One consequence may be seasonal depression, characterized by low mood, fatigue, and general lethargy.

Vitamin D deficiency can also weaken the immune system, increasing susceptibility to colds and influenza. Therefore, supplementation during winter is particularly important to maintain adequate immune protection.

Vitamin D and omega-3 combination

Vitamin D intake may be more effective when combined with omega-3 fatty acids. Omega-3s support the absorption and utilization of vitamin D in the body, so it is advisable to take vitamin D supplements together with fish oil or omega-3-rich foods.

Natural sources of vitamin D

Vitamin D is primarily found in animal-based foods, including:

• Cod liver oil: 142 µg per teaspoon
• Herring: 25 µg per 100 g
• Mackerel: 24 µg per 100 g
• Oil-packed sardines: 9 µg per 100 g
• Tuna: 6 µg per 100 g
• Milk: 3 µg per 100 g
• Whole grains: 3 µg per 100 g
• Egg yolk: 1 µg per 100 g
• Liver: 1 µg per 100 g

To ensure adequate vitamin D intake, these foods should be consumed regularly, particularly during winter when sunlight exposure is insufficient.

Vitamin D is essential for many bodily functions, especially maintaining bone health and proper immune function. A large portion of the Hungarian population becomes deficient during winter, making supplementation crucial. To ensure adequate intake, a balanced diet may be complemented with vitamin D supplements, preferably combined with omega-3 fatty acids. Taking proper care of oneself during winter is essential—and vitamin D plays a key role in this process.